Fix blacklists update when the clock goes backward

This fixes a possible corner case when the wall clock is set
back in time, which could cause the blacklists not to be
updated.

Closes #413

app/src/main/java/com/emanuelef/remote_capture/Blacklists.java

@@ -21,6 +21,7 @@
 
 import android.content.Context;
 import android.content.SharedPreferences;
+import android.os.SystemClock;
 import android.util.ArrayMap;
 
 import androidx.collection.ArraySet;
@@ -56,7 +57,7 @@
  */
 public class Blacklists {
     public static final String PREF_BLACKLISTS_STATUS = "blacklists_status";
-    public static final int BLACKLISTS_UPDATE_SECONDS = 86400; // 1d
+    public static final long BLACKLISTS_UPDATE_MILLIS = 86400 * 1000; // 1d
     private static final String TAG = "Blacklists";
     private final ArrayList<BlacklistDescriptor> mLists = new ArrayList<>();
     private final ArrayMap<String, BlacklistDescriptor> mListByFname = new ArrayMap<>();
@@ -66,11 +67,13 @@ public class Blacklists {
     private boolean mUpdateInProgress;
     private boolean mStopRequest;
     private long mLastUpdate;
+    private long mLastUpdateMonotonic;
     private int mNumDomainRules;
     private int mNumIPRules;
 
     public Blacklists(Context ctx) {
         mLastUpdate = 0;
+        mLastUpdateMonotonic = -BLACKLISTS_UPDATE_MILLIS;
         mNumDomainRules = 0;
         mNumIPRules = 0;
         mContext = ctx;
@@ -110,11 +113,15 @@ public void deserialize() {
         String serialized = mPrefs.getString(PREF_BLACKLISTS_STATUS, "");
         if(!serialized.isEmpty()) {
             JsonObject obj = JsonParser.parseString(serialized).getAsJsonObject();
-
             mLastUpdate = obj.getAsJsonPrimitive("last_update").getAsLong();
             mNumDomainRules = obj.getAsJsonPrimitive("num_domain_rules").getAsInt();
             mNumIPRules = obj.getAsJsonPrimitive("num_ip_rules").getAsInt();
 
+            // set the monotonic time based on the last update wall clock time
+            long millis_since_last_update = System.currentTimeMillis() - mLastUpdate;
+            if (millis_since_last_update > 0)
+                mLastUpdateMonotonic = SystemClock.elapsedRealtime() - millis_since_last_update;
+
             JsonObject blacklists_obj = obj.getAsJsonObject("blacklists");
             if(blacklists_obj != null) { // support old format
                 for(Map.Entry<String, JsonElement> bl_entry: blacklists_obj.entrySet()) {
@@ -179,7 +186,7 @@ private void checkFiles() {
 
             if(!f.exists()) {
                 // must update
-                mLastUpdate = 0;
+                mLastUpdateMonotonic = -BLACKLISTS_UPDATE_MILLIS;
             }
         }
 
@@ -200,9 +207,9 @@ private void checkFiles() {
     }
 
     public boolean needsUpdate(boolean firstUpdate) {
-        long now = System.currentTimeMillis();
-        return((now - mLastUpdate) >= BLACKLISTS_UPDATE_SECONDS * 1000)
-                || (firstUpdate && (getNumUpdatedBlacklists() < getNumBlacklists()));
+        long now = SystemClock.elapsedRealtime();
+        return (((now - mLastUpdateMonotonic) >= BLACKLISTS_UPDATE_MILLIS)
+                || (firstUpdate && (getNumUpdatedBlacklists() < getNumBlacklists())));
     }
 
     // NOTE: invoked in a separate thread (CaptureService.mBlacklistsUpdateThread)
@@ -232,6 +239,7 @@ public void update() {
         }
 
         mLastUpdate = System.currentTimeMillis();
+        mLastUpdateMonotonic = SystemClock.elapsedRealtime();
         notifyListeners();
     }